It is common in the electronics art to form electronic assemblies containing many different electronic components attached to a printed circuit board (PCB). The PCB is frequently formed from a paper, cloth and/or glass reinforced plastic substrate to which metal foil has been applied. The metal foil is etched or otherwise delineated to create conductive metal interconnection lines and bonding or mounting pads for the various components.
In dense surface mount PCB construction, the attainable power dissipation of the components is often severely limited by the combined thermal resistance of the surface mount component package and the PCB. For example, present day low cost surface mount transistor packages have a thermal resistance in the range of about 45.degree. C./watt for an SO-8 type package to about 220.degree. C./watt for an SOT-23 type package. SO-8 and SOT-23 are standard package outline designations well known in the art.
In determining the overall thermal resistance from the electronic device, e.g., a transistor die, to the heat sink where the heat is absorbed, the thermal resistance of the PCB must be added to that of the package. The PCB can add a large amount of thermal resistance. High thermal resistance is undesirable since it results in higher component operating temperatures for the same power dissipation, which in turn shortens the operating life of the component. The above-described problem is especially severe for electronic assemblies operating in the radio frequency range, e.g., above about 1 Megahertz and particularly above about 100 Megahertz. The higher the desired operating frequency the more difficult the problem since, in general, the negative impact of parasitic capacitance and/or inductance increases with operating frequency. To be useful at high frequencies, the solutions to the thermal dissipation problem must be low in all parasitics.
In the past, various attempts have been made to reduce the thermal resistance of components mounted on PCB's. For example, ceramic materials such as alumina or beryllia are used for the PCB substrate, but they have the disadvantage of being very expensive compared to conventional reinforced plastic type PCB's. Alternatively, stud mounted electronic components are used where the electronic chip or die is mounted on a metallic stud that passes through a hole in the PCB and attaches directly to an underlying heat sink. However, stud mount packages are not useful for surface mount assembly and it is expensive to electrically isolate the die from the stud or the stud from the heatsink.
Alternatively, a hole may be provided in the PCB through which a portion of the external heat sink protrudes so as to come into direct contact with the individual device or component package mounted on the PCB. However, this arrangement is not useful with many different types of components and often does not provide sufficient heat removal from plastic encapsulated components. In a still further approach to improve heat removal, the entire PCB assembly may be immersed in a coolant fluid, but this requires expensive exterior packaging and fluid circulating means.
None of the foregoing prior art techniques are suitable for electronic assemblies when very low cost is an important consideration, and particularly when surface mount assembly is desired. Accordingly, it is an object of the present invention to provide a means and method for low cost electronic assemblies having improved power dissipation capabilities and/or lower operating temperature.
It is a further object to provide improved power dissipation capabilities and/or lower operating temperature in electronic assemblies utilizing PCB's, especially reinforced plastic based PCB's.
It is an additional object to provide improved power dissipation capabilities and/or lower operating temperature in electronic assemblies adapted for surface mount assembly, especially those adapted for automated assembly using machine placeable components.